This invention relates to telephone ring and Caller ID detector circuits.
So-called xe2x80x9csmart telephonesxe2x80x9d are often provided with circuitry which decodes a Caller-ID signal sent by a central office between the first and second ring signals, so as to identify the calling party prior to answering the call. Typically, frequency-shift keying (FSK) encoded caller ID data is transmitted between first and second ring signals. The called party""s telephone extension unit includes a detector, which demodulates the FSK signal and allows display of the Caller-ID before the called party answers the call.
U.S. Pat. No. 5,796,815 (Guercio et al.) discloses a communications device that is adapted to detect a ring and a Caller ID signal with the same detection circuitry. The communications device includes an off-hook detector coupled between a telephone line and a communications circuit. When the telephone is on-hook, an electrical resistance of at least 25 Mxcexa9 exists between two switch terminals of the off-hook detector, thereby effectively d.c. decoupling the telephone. A capacitor connected between the two switch terminals of the off-hook detector couples a.c. signals from the telephone line to the communications circuit when the telephone is on-hook. The ring signal has an amplitude of 100 volt peak to peak and a frequency in the range of 20 Hz. The Caller-ID signal has an amplitude of approximately 1 volt peak to peak and employs FSK using frequencies in the range of 1,200 Hz and 2,200 Hz to encode logic xe2x80x9c1xe2x80x9d and logic xe2x80x9c0xe2x80x9d respectively.
The value of the coupling capacitor is selected so that the amplitude of the ring signal is attenuated to approximately 1 volt peak to peak, this being the maximum allowable input voltage range of commonly available A/D converters. On the other hand, the amplitudes of the Caller-ID FSK frequencies are changed by no more than 5%. The signal amplitudes of both signals are thus brought into an overlapping range, allowing both signals to be detected with a single detector according to their respective frequencies.
There are several drawbacks with such an arrangement. First, the input impedance to the communications circuits series is substantially equivalent to a series combination of an inductance and a resistance. Together with the coupling capacitor, this forms an R-L-C high pass filter whose output is thus frequency dependent. The frequency of the ring signal varies from one country to another, typically lying within a range of 15 to 80 Hz. Thus, at the higher frequency range of the ring signal, the lower frequency of the FSK encoded Caller-ID signal is only fifteen times larger than the ring signal frequency. It is not possible, to attenuate an 80 Hz signal sufficiently using the coupling capacitor proposed by Guercio et al. to the same extent as may be done for a 20 HZ signal. Thus, effective translation of the ring and Caller-ID signals at all frequencies into the permitted range is impossible.
It is an object of the invention to provide a detector for detecting high voltage ring signal and the low voltage Caller-ID signal, wherein the drawbacks associated with hitherto-proposed circuits are eliminated.
According to a first aspect the invention, there is provided a method for distinguishing between a first voltage signal having a high amplitude and a second voltage signal having a low amplitude, said first and second voltage signals comprising respective AC components superimposed on a DC level, the method comprising the steps of:
(a) blocking the DC level of the first and second voltage signals,.
(b) attenuating the respective AC components of the first and second signals by an attenuation factor which is voltage dependent and substantially frequency independent, so that the respective AC components of the first and second voltage signals after attenuation have similar amplitudes, and
(c) processing the attenuated respective AC components of the first and second voltage signals so as to distinguish between the first and second voltage signals according to their respective frequencies.
According to a second aspect of the invention, there is provided a circuit for distinguishing between a first voltage signal having a high amplitude and a second voltage signal having a low amplitude, said first and second voltage signals comprising respective AC components superimposed on a DC level, the circuit comprising:
a capacitor for blocking the DC level of the first and second voltage signals,
an attenuator for attenuating the respective AC components of the first and second signals by an attenuation factor which is voltage dependent and substantially frequency independent, so that the respective AC components of the first and second voltage signals after attenuation have similar amplitudes, and
a processor for processing the attenuated respective AC components of the first and second voltage signals so as to distinguish between the first and second voltage signals according to their respective frequencies.